This invention relates generally to feedthrough capacitor filter assemblies, particularly of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. More specifically, this invention relates to an improved feedthrough capacitor filter assembly of the type incorporating a hermetic seal to prevent passage or leakage of fluids through the filter assembly, wherein a leak detection passage is provided to accommodate and facilitate post-manufacture and pre-usage testing of the hermetic seal.
Feedthrough terminal pin assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators and the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage of electrical signals from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of patient body fluids into the medical device housing, where such body fluids could otherwise interfere with the operation of and/or cause damage to internal electronic components of the medical device.
In the past, two primary technologies have been employed to manufacture the hermetic seal. One technique involves the use of an alumina insulator which is sputtered to accept brazing material. This alumina insulator is brazed to the terminal pin or pins, and also to an outer metal ferrule of titanium or the like. The alumina insulator supports the terminal pin or pins in insulated spaced relation from the ferrule which is adapted for suitable mounting within an access opening formed in the housing of the medical device. In an alternative technique, the hermetic seal comprises a glass-based seal forming a compression or fused glass seal for supporting the terminal pin or pins within an outer metal ferrule.
The feedthrough terminal pins are typically connected to one or more lead wires which, in the example of a cardiac pacemaker, sense signals from the patient""s heart and also couple electronic pacing pulses from the medical device to the patient""s heart. Unfortunately, these lead wires can act as an antenna to collect stray electromagnetic interference (EMI) signals for transmission via the terminal pins into the interior of the medical device. Such unwanted EMI signals can disrupt proper operation of the medical device, resulting in malfunction or failure. For example, it has been documented that stray EMI signals emanating from cellular telephones can inhibit pacemaker operation, resulting in asynchronous pacing, tracking and missed beats. To address this problem, hermetically sealed feedthrough terminal pin assemblies have been designed to include a filter capacitor for decoupling EMI signals in a manner preventing such unwanted signals from entering the housing of the implantable medical device. See, for example, U.S. Pat. Nos. 4,424,551; 5,333,095; 5,751,539; 5,905,627; 5,973,906; and 6,008,980.
While feedthrough capacitor filter assemblies have provided a significant advance in the art, one potential area of concern is that the filter capacitor is often incorporated into the terminal pin assembly in a way that can mask a defective hermetic seal. More particularly, a defective braze or a defective glass-based seal structure, which would permit undesirable leakage of patient body fluids when mounted on a medical device and implanted into a patient, can be obstructed by the mounting of the filter capacitor and its associated electromechanical connections. For example, with reference to the feedthrough filter capacitor shown in U.S. Pat. No. 4,424,551, a ceramic filter capacitor is bonded to a glass seal and then embedded in epoxy material. Typical post-manufacture leak testing is performed by mounting the feedthrough assembly in a test fixture, and then subjecting one side of the feedthrough assembly to a selected pressurized gas such as helium. While the bulk permeability of the epoxy material is such that helium under pressure can penetrate therethrough in the presence of a defective hermetic seal, the duration of this pressure test (typically a few seconds) is often insufficient to permit such penetration. Accordingly, the epoxy material can mask the defective hermetic seal. The thus-tested feedthrough assembly can then mistakenly be incorporated into a medical device and implanted into a patient, wherein slow leakage of patient body fluids through the feedthrough assembly can cause the medical device to malfunction or fail.
The present invention is directed to an improved feedthrough capacitor filter assembly suitable for use in an implantable medical device or the like, wherein the feedthrough assembly includes a leak detection passageway for accommodating and facilitating post-manufacture hermetic seal testing.
In accordance with the invention, an improved feedthrough capacitor filter assembly is provided for use in human implant applications and the like, such as in a cardiac pacemaker or defibrillator, wherein the filter assembly includes a leak detection passage for facilitated hermetic seal testing subsequent to manufacture and prior to use.
In one form, the feedthrough capacitor filter assembly comprises a capacitor having first and second sets of conductive electrode plates embedded within an insulative or dielectric body such as a monolithic ceramic body. At least one feedthrough terminal pin extends through the capacitor in conductive relation with the first set of electrode plates. An outer ferrule is mounted about the capacitor in conductive relation with the second set of electrode plates. A hermetic seal formed typically of an alumina insulator or a fused glass dielectric material is seated within or over the ferrule at one side of the capacitor, in hermetically sealed relation with the ferrule and the at least one terminal pin to prevent leakage of fluid, such as patient body fluid in a human implant application, through the filter assembly. A leak detection vent is formed in the assembly to accommodate and facilitate post-assembly fluid leak testing of the hermetic seal, as by subjecting the hermetic seal to a selected pressurized test gas such as helium or the like, prior to implantation of the filter assembly in a medical device into a patient.
In several of the preferred embodiments, the hermetic seal and the capacitor are separated by a short gap by a dissolvable washer. This gap provides a leak detection vent which facilitates detecting the presence of the test gas leaking past the hermetic seal. In other preferred embodiments of the invention, a leak detection passage is provided through the capacitor which further facilitates detecting the presence of the test gas leaking past the hermetic seal into the gap.
Other features and advantages of the invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.